This invention relates to improvements in communication systems of inductive-carrier type and, more particularly, this invention relates the use of such communication systems for the transmission of the sound portion of motion pictures or the like in drive-in motion picture theaters directly to the radios of automobiles parked within the theater area.
The drive-in motion picture theater is a theater designed for the showing of motion pictures wherein the audience drives their automobiles into the theater area and parks in designated parking spots in view of a large screen. Normally, sound is transmitted to each car by means of portable speaker which is detachably mounted on a stanchion post near such designated parking spot. The speaker is removed from the post and placed in each automobile during the viewing of the film. Sound signals are transmitted by means of underground wires from the projection booth to each post, through wires in each post and thereafter through wires from the post to the speaker. The signal is transmitted from the sound amplifier in the projection room to the speaker in each automobile in much the same manner as a signal is transmitted in a home sound system.
There are many inherent disadvantages to such a system. Each car must be exactly positioned in close proximity to the speaker posts. Each of the individual speakers must be maintained by the theater. It is common for cars to attempt to leave the theater at the end of the show without removing the speaker and thus cause damage to the post, the speaker and the car itself. Additionally, drivers commonly hit the speaker posts by accident during manuevering in parking. As can be seen from the above, the maintenance of such an exposed system is comparatively expensive. Also, the sound quality is much inferior to that in enclosed theaters because of the use of very small speakers employed in drive-in theaters in order to accommodate the speakers to the stanchion posts that support the speakers.
As will be more fully discussed hereafter, the present invention comprises the use of an inductive-carrier electro-magnetic transmission system of localized type whereby the sound portion of the motion picture is transmitted to the car radios of the automobiles parked within the theater area. Many systems of the inductive-carrier type, including those of the applicant, have been employed in the past for localized restricted-range transmission on highways, at airports and on railroads. To applicant's knowledge no such system has ever been adopted and employed for use with automobile radios in drive-in motion picture theaters and the like. The past uses of such inductive-carrier type systems have presented serious technical problems when operated without license under the low-power rules of the Federal Communications Commission (FCC) of the United States and when using relatively-high carrier frequencies, such as those in the standard (medium wave) broadcast band. Radiation of electrical wave energy which is an inherent characteristic of inductive-carrier systems when operated at radio frequencies, often extends over distances far in excess of the permissible field-strength limit specified by the U.S. Government and others for unlicensed radio devices of restricted-range type. While it has been possible, by careful adjustment of radio frequency (r.f.) carrier level to comply with the Government's rules in certain localized applications, such as the highway radio system installed by the applicant on the George Washington Bridge, New York City, in 1940, experience in most cases has demonstrated that it is extremely difficult, and in some instances impossible, to comply with the Government's rules over any substantial period when unattended transmitters are employed and, at the same time to maintain a sufficiently strong induction field at broadcast frequencies to enable good reception in radio-equipped cars traveling over lengths of highway served by the system.
Experience with roadside conductors of various types, including single and dual-conductor transmission lines has indicated that the strength of the induction field about these conductors is subject to substantial variation along their length. Near the transmitter source, for example, the field strength may be too high to comply with the Government's rules at broadcast frequencies if a strong, noise-free signal is to be provided in the desired reception area. In addition, if the cable is installed below the surface of the ground as would be required for use in drive-in theaters where above-surface installations would not be desirable, variations in the inductive-signaling field due to changes in soil conductivity under different weather conditions and other irregularities in environmental conditions would present difficulties over a substantial period of time in maintaining a reasonably-constant field strength and restriction of radiation within limits set by the Government.
Moreover, experience with conventional forms of cables, or wires, when employed as r.f. signal conductors for the purpose of producing an induction-signaling field as a means of impressing carrier-signal energy on the antenna system of radio brodcast receivers carried by motor vehicles indicates that the coupling loss between the vertically disposed vehicle antenna and the horizontally-polarized signals from the cable system is unnecessarily high. This results in requirement of substantially more r.f. power in the cable system than would be required if a convolutive field, having vertical and horizontal polarization characteristics, were provided. The present system as employed in drive-in theaters incorporates as an important element what are believed to be unusual and novel means for developing such a convolutive field to produce a signal of maximum strength in receiving systems of motor vehicles carrying conventional antennas. This, in turn, assists in meeting the requirements of the Government with respect to restricted-range radio devices.